Method and means for transmitting a plurality of signals over the same channel



METHOD AND MEANS FOR TRANSMITTING A PLURALITY oF SMITH Feb. 3,

SIGNALS OVER THE.SAME CHANNEL Filed May 3, 1959 2 Sheets-Sheet 1 INVENTOR. Jimi: f. .mr/7H ATTORNEY Feb. 3, 1942 2,271,721

J. E. SMITH METHOD AND MEANS FOR TRANSMITTING iPLURALITY oF SIGNALS OVER THE SAME CHANNEL Filed May 5, 1939 2 Sheets-Sheet 2 'thfinterrupted tone current Paiemed Feb.,3,`19422 i METHOD .AND MEANS Fon 'r-RANSMITTING A4r PLURALITY .or SIGNAns Aoven. THE

sAME CHANNELc i.,

" Y v'llames Ernest Smith, Brooklyn,

' Radio Corporation-of America, a

Delaware u N. assi'gnor to corporation-of f AppueationMyaisss, serial-No. 27m69- 9 claimsl (c1. 17a- 51) This invention relates to radio signaling and particularly to radio telegraphicfsignaling in which a plurality of messages are sentover thev same Wire or radio link. The inventionl is ca# pable of other uses butfor purposes of explanation it will be disclosed in connectionl with a radio 5 telegraph system in Wlfiichfafplurality of signals are sent over conducting wires. to; a radio trans; mitting station where the signalsmay vbe transmitted by radio todistant points. In a, system of this vsort the point l transmission is usually locatedline a large city and the transmitting station is a considerable distance away. The signals are transmitted yover wire lines from the central oflice to thetransmitting station in various formsrof multiplexing,V one being to convert theI D.YC. mark .and space signals Iinto interrupted alternating. current that is referred to as tone frequencies because the frequency is sufficiently low to be heard in a telephone head set.

rTo send aplurality of signals Vover the line con--v nection to the radio station, a plurality of tone frequencies have been used with appropriate -band passlters to separate the tone frequencies at thefradio transmitting station. kWhen the wireline is used as a single channel, for` example,

value so as to be well aboye the noise level;kv As Vthis same conductor -is progressively loaded with signals from additional channels-all jtone generators will have the current.progressivelydecreased" and finally when the maximulriv number of chanof origin in the signal.,

Wheatstone, Creed or ot Fig., 3 is a s'et of. graphs'showing the volta-.gef-Vlr frequency characi'eri'stics of lters used withg varying" amplitude and varying frequency signals.

Referring' to' the drawings, in Fig'. 1', l Aindicates a perforated tape y her type. Thesejdevices are well, known in ,thev art and have long been used for transmitting telegraphsignals so thev transmitter* I" is shown" 'only in diagrammatic form as consisting cfa switch'memberl caused to vibrate alternately against positive conti'iictv 3 and negative contact '4 by well known perforated tape mechanisrn", notv shown.' Transmitting del' vice 2 may be af duplicate 'of device I'in which the switch'member 5 alternately contacts positive contact 6 and .negative Contact 1. Battery l8 is illustratedas thesourceA of voltage supply for thek devices land 2. Polar Vrelay coil 9 is" co'n.

nected to switch 2 andwhen'energized moves the switchj tongue 'I0 against either' contact Il or contact I2,'de'pending[upon the polarization chosen'and the direction ofV Icurrent through the coil. Relay coil I31is similarly `connected to switch 5 and operatespolar tongue Il. Contact I5 in this relay isthe only one that conducts current, lvb'eing a dead contact. .The illustrawill be of I,optimum tion' shows,V the polarization-suchthat contag'stsl 3,12, 6 and I6 are mark contactsand contacts 4', Il', 1 andlS are space contacts.'

The' tone' generator could nbe an alternatingv current generatorof any type but for convenience l ofcontrol I prefer tofusefthe beat typeof oscilnels is routed over one conductor the tone generator current on each channel will be decreased level, which of course is quite objectionable.- It is an object of my invention to overcome this difiiculty by providing ,meansy for Y increasing the wire line signals by interrupting fthe' tone generator Lfor one set of signals while varying .the frequency of that same signalcurrent for another set of signals. Y -V Another object of the invention is to' increase the wire linemessages without reducing` the` wire line current.

Other vobjects will appear in the -following description, reference beinghad to the drawings in which: Y Fig. 1 is a part block illustration of a transmission system for sending a plurality of messages in accordance with my invention.

Fig. 2 is a set of graphs illustrating the. principles of the invention. l

part diagrammatic illustration and lator where the tone frequency is produced by beating together rof two vacuum tube oscillators.

lReferenceclzharacterl I1 generally indicates one oscillator which. may havey any desired frequency, for'example, '75 kilocycles, and the other one,y differing therefrom by two beat frequencies, say .8 kilocyclefand .'1*l kilocycle. In this case condenser IQ is connected in fixed relation -with the inductance 20v so asto determine the frequency of I15.8 kilocyclesinthe oscillator i8. VCondenser 2| is.4 adapted to .be'connected inparallelwith condenser i9 .by switch tongue -which will chang'ethe frequency to 75,7, f- Y The plate supply ofoscillators l1 and I8 'is indicated by terminals 22, 23.- The voltage may be v of any` desired value.. A negative biasmay bey placed on these oscillators of any sort, but I have illustrated by way of example resistance. having bypass condenser 26for this purpose. Conydensers 21 and 28 and resistances 29 andv 30 are connectedy tothe input andy outputgelectrode throughv coil 2l)v to constitute the well known Hartley oscillating circuit. Resistance 30 of transmitting device ofr the stage on the space.

"if desired, a threshold bias the rectifier passes into smoothing filter 53.

The operation of the embodiment disclosing4 grid 3I and resistance 30 of oscillator I8 is similarly connectedv to control `grid 32 of a mixing tube 33. This tube may have the usual plate and screen grid supply of vsuitable voltages and an appropriate bias typified by resistance 34, as well as a suppressor grid {connected to the filament..

The plate circuit of tube 33 feeds into the input coil 35 of a push pull keying-stage indicated generally at 36.

Tape transmitter Iis adaptedto apply a negative bias 31 to this keying stage on the signal mark and a greater bias by adding an additional battery 38 on the space of the signal, or the reverse if desired. 'I'his bias is not of'` the' usual beat produced will have this value.

2,271,72i A oscillator I1 is adiustably connected to control for example, to'alternately engage contacts 3, 6, and 4, 1, onthe mark and spacey parts of the signal. 1f no B vsignal is being transmitted the tone frequency will be, in the example given, 800 cycles because condenser I9 will be the only one in circuit with coil 20 of osci1lator=I8 and the Therefore,

`as the A messageis being sent the tone frequency value that would block the grids ofthe keying 1t has Asufficient negative bias value to permit some current to flow through the plate circuit of the 'push pull tubes on the space part o f Athe signal. vTlfiejamountpi current allowed to thus flow through may be varied but I have found it satisfactory to permit of the marking value of current to' fiow on the spaces of the signalfor a purpose that will be later apparent.

The output of keying stage 36 passes into insignal producedby the shifting of the bias on keying stage 36 will produce an alternating current signal of a single frequency of 800 cycles for both marks and spaces as indicated by graph C in Fig. 2. In this stage ,there is still 20% current atv 60 for'the spaces; VReference character 6I indicates the mark of the signal, though of course the reverse could be the case as it is common practice to reverse signals. Band pass filter 40 put coil 39' of aband pass filter 40 of the usual.

construction. Similar band pass filters 4I and 42 may pass signals of channels II and III. These will have frequencies. differing from each other and from the frequencies of channel I and they may b e equipped as in' channel 'I foram-4 plitude and frequency modulated signals if desired. Any number of these channels maybe used and I have shown three only by way of example. All the channels unite at the input 43 of amplifier 44 to'boost the signals to any desired value for transmission over the long Vlines 45 to the radio transmission station indicated generally at 46. At the transmission station the signals may be further amplified by the usual type of amplifier 41. The output of this'amplifier contains all the signals. I

The signals of channel I, both the A message and B message, passI through band pass filter 43 and themessages of'channels II and IIIY pass through band pass filters 49` and 5 0.resp ectively."

f Since the-invention may beexplained irrcoiinec-V tion with channel I, the` apparatus for'channels`- II and III is not further illustrated.

The output lines 5I of band pass filter confV tain, as previously indicated, the AA and B signals. Rectifier 52 connected to input lines 5I converts voltage therefrom into direct current voltage. Bias 53 causes this rectifier to pass current only above apredetermined threshold value. Filter 53 may be of the usual construction for smoothing out the alternating components.`

The'B signal which'is frequency modulated, is passed throughA voltage limiter 54 of any desired type.A These limiters are well'known in the art and the details are therefore not illustrated; 'I'he output of the limiter passes into input coil 550i a frequency converter or demodulator consisting of a plurality ofseries coils 58 and a plurality of shunt condensers 51. This arrangement is the well known type of low pass filter. The Output of the converter passes into rectifier 58 having, 53 and the output of my invention will now'be described.

Referring to Fig. l, perforated tapes not illustrated containing the A and B signals will oper-'- ate the pecker pins typified by switches 2 and 5,

passes the signal currentjof the keying stage 36 in accordance with graph D in Fig. 3, f1 indicating the frequency at vwhich the A signal is being sent when no B signal is being transmitted.V

If Aa. BV signal is vtransmitted simultaneously with theA signal, pecker pins typified by switch .5 willbeoperated by the perforated" tape, not

shown, to alternately engage contactsi and 1. This will energizepolar relay I3'and alternately move the switch 'I'.onluer |45 between live contact I5 and dead fcontact I6. This, in accordance with the assumption previously made, will produce a frequency of 75.8` cycles for vthe mark of the signal by including condenser I9 alone in the oscillating circuit of the input of `oscillator I8 when contact I6'is engaged.

When contact I5 is engaged for the space of 'the signal the inclusion Vof condenser 2| in the .oscillating circuitof the'input of' oscillator I8 will cause this oscillator `tclY have a `frequency .of '75.7 kilocycles. Therefore, message B will be transmittedby a varyingbeat frequency having 800 cycles fortthe mark and ',700 `cycles for the space ofthe signal. -If no A' signal'. were being transmitted the mark and space would be of uniform amplitude but when the A signal is being v transmitted thecombined'A and B signals will be indicated by graph E of Fig; 2.l Here as in graph C, reference character 3l and 0I- will indi-V cate thefcpace iandmarkf signals. respectively of the A` inessag'efbythedierencein amplitude of the cun'ent-.iflnthi'sfcase the..Afftransmitter` I i varies the {almp'litixtie of atonegcurrent havingff first a frequencybfWOO'fjand-.their a'lfrequency' of 800. As is illustratedlin;the'fdrawinga'the space is one v-frequenc-.y andthe mark'another frequency, but this is" fcriillustrationv purposes only.` The vfrequency -ofthe "tone generator might just aswell changeat any vother points of the keyingin the A'signal. Y v

The signal of tlegeneral form indicated in graph Elwill be combined with other signals of channels 2 and), if 'any,for even with a still greater number of* channels. The combined currentA will be amplifiedby the' usual amplifier 44,"

afterwhich it will be transmitted vover the long` lines, lto the transmittingstation orftogany other point. 'A 1 At the transmitting" rent may be further ampli'ed at 41 Aandthe combined voltages appear at the inputs of all of the,

band pass filters 43, 49 and 5 0. Band pass lter 43 will be adjusted to pass onlythe varying beat frequency and to exclude the frequenciesb of tone frequencies, variable or not, of channels II and III. The voltage frequency characteristic of the band pass filter 4l is indicated by curve D in Fig. 3. This band pass filter is of suiiicient width,

statiouthe combined curindicate two messages, two utilization circuits, a'

figuratively srvealringto pass the frequencies l700 cycles and 800cycles indicated at .f1 and `fo in Fig. 3. v

The amplitude-modulated variable frequency voltage wave of graph E in Fig.'2 will appear at the input of both rectifier 52 and limiter A54. By arrangingtherectier 52 either by an appropriate bias 53` or by other means known in theart, current above the 1ine.62 in Fig. 2 only, will be` passed. This will eliminate the pulsating current 60 iny the spaces., The output of rectifier '52 passes preferably through a filter 53' to smooth out the alternating component in the marking indication 6| of the signal.` The pulsating unidirectional wave in the output of the lter'53 can then beinterpreted in any way, either recorded as a received message in a wire or cable telegraph system, or used to modulate a radio transmitter for transmission to distant points. This part of the system is not a part 4of my invention and is not described.A A

The limiter 54 which may be of any well known" type, for example,as shown in my joint application with James N. Whitaker and Gilbert R.

Clark, Serial No. 270,332, filed April 27, 1939, may

be used for this purpose. vThis limiter will be so adjusted that current values greater than the maximum value of the spacing current 60, will be cut off, for example, the limiter may be adjusted to cut ofi the signal at the lines 63. Alternating current of uniform maximum amplitude will thus I be introduced into secondary 55 of the frequency converter, or demodulator, and the amplitude of the voltage in the output of this converter will vary with the frequency. As the frequency increases the voltage will decrease, as shown by curve G of Fig. 3. Therefore, the voltage at the terminals of the rectifier 58 will be amplitude modulatedv and will have highvand low portions for the marks and spaces. The output of the rectifier may be passed through'a filter 53' to smooth out the alternating components. By use of a threshold device such as bias 53, yfor example, the output of the rectifier may be made to have current only for the marks of the signal and the output of the rectifier 58 will be of the same character as the output of rectifier 52. This may be interpreted as av message at the final receiving vstation of'a wireSyStm 4or may be used l' to modulate a radio transmitterfor transmission to distant points through kthe air. It will be kapparent that my improvement can be. used in strictly wire line communication or in a radio system.

Instead of using a low pass filter I may shift the sub-carrier frequency to one side of the channel filter to position fa in Fig. 3. The lower frequency value ofthe beat would then be, ,f3 and the upper frequency value would be f4. The band pass filter would perform a double duty by acting also as a converter or demodulator. With this arrangement only one side band of the amplitude modulated A Vsignal would pass through the band pass filter. 1

Instead of using low pass filters for a converter I also may use high'pass filters. The voltage frequency characteristic in this case would be indicated by reference character H. Various other modifications may be made without departing from the spirit of the invention. Having described my invention,

1.*In a telegraphreception, a link having a signal wave with its frequency and its amplifrom a higher to a lower value to tude changing what I claim limiter connected between' said link and oneof said circuits Afor passing current only above the lower amplitude value and a limiter connected between said link kand the other of "said circuits'for passing current only below saidamplitude value` 2. In a` telegraph reception, a link having a signal wave with:V its frequency'an'd its amplitude changing from a higher to a lower value to indicate two messages, twoutilization circuits, a

limitingrectifier connectedbetweenjsaid linkl and one of saidcircuits for passing current thereinto only above fthe lower amplitude value' and a limiter connected *between said link and the otherA of said circuits for passing current thereinto only below saicLamplitude value.

- output anclmeansv connected'to-said limiter vto convert the frequency changes in its output' into amplitude modulation with two amplitude values. y 4. In telegraph reception, a link having a signal wave with its'frequency and its amplitude changing from a higherto a lower value to indicate two messages, a limiting rectifier connected to said-link for passing currentonly above the lower amplitude value, a limiter connected to said link for passing current only below said amplitude value, a filter connected to said rectifier to eliminate the frequency changes in its output, means connected to said limiter to convert the two frequency values in its output into amplitude modulation'with two amplitude values and a limiting rectifier connected to said means for passing current only above the lowest of the lastmentioned values.'

5.' In telegraph reception, a link having a signal wave with its frequency and its amplitude changing from a higher to a lower value to indicate two messages a limiting rectifier connected yto saidl link Vfor passing current'only above the` .lower amplitude value,f ay limiter 'connected to -said link for passing current only below said amplitude value, a-f-llter connected to saidV rectifier to eliminate the frequency/changes in its output, means connected to said limiterto convert the two frequency values in its output into amplitude modulation with two amplitude values, a limiting vrectifier connectedto said means for passing curtude varied between two finite values to produce mark and space signals of one telegraph message and the frequencyof the wave varied to produce mark and space signals of another n-essage, the steps in the methodwhich consists in passing wave energy of one'sign only above the lower of said values into one circuit to eliminate the frequency variations of the second message in the spaces of the first message and passing wave energy below said lower value into another circuit to eliminate the amplitude variations of the first message.

7. In the reception of a wave having its amplitude varied between two finite values to produce mark andspace signals of one telegraph message and the frequency of the .wave varied to product mark and space signals of another message, the steps in the method which consists in rectfying and passing wave energy above the lower of said valuesinto one circuit to eliminate the Vfrequency variations of the second message in the spaces of the first message, filtering the passed energy to smooth out the frequency variations in the marks, passing wave energy below said lower value into another circuit to eliminate' the amplitude variations of the first message and converting the frequency variations of the lastmentioned energy into amplitude variations.

8. In the reception of a wave having its amplitude varied between two finite values to produce mark and space signals of one telegraph message and the frequency of the wave varied to produce mark and space signals of another message, the steps in the method which consists in rectifying and passing wave energy, against a counter elec'- tromotive force equal to. the lower of said values to eliminate the frequency variations of the second message in the spaces of the first message, filtering fthe passed energy to smooth out the frequency variations in-the marks, passing 4wave energy below said lower value intoanother circuit to eliminate the amplitude variations of the rst message and converting the frequency variations of the last-mentioned energy into amplitude variations. Q

9. In the receptionr of a wave having V'its amplitude varied between two finite values to produce mark and space' signals ofone telegraph message and the frequency of the wave varied to produce mark and space signals of another message, the method which consists in rectii'ying and passing wave energy above the lower of said values into Ione circuit `to eliminate the frequency variations of the second message' in the spaces of the first jmessage, lteringthe passed energy to smooth value'to eliminate the amplitude variations of the rst message, converting the frequency varia- V tions of the last-mentioned energy into amplitude `variations and indicating the last-menl tioned variations to reproduce the second message. 'Y

` JAMES' ERNEST SMITH.- 

